That's why Staff gives you a +1 AC all the time, which is nearly as good as a small shield, in addition to the encounter power that's nearly as good as the Shield spell.
The staff power is much better than the shield spell.
First of all, its unbound, and as you get better it gets better.
Second of all, it works on any defense, fort, ref, will, or AC. Shield only works on AC.
Third of all it works after the enemy rolls damage where as Shield works after the hit. Which means that if the enemy rolls damage poorly you can take the hit and keep going.
A wizard that starts with 16 con and pumps it each and every chance will have +7 to all defenses once/encounter after damage has been rolled. Considering most enemies will be hitting ~50% of the time that pretty much takes any hit and turns it into a miss.
ED: Orb looks good on paper until you realize that you have to hit with a daily to get use out of it. And that means that you are less likely to get value out of it on the enemy you want. Especially if you don't end up using a daily in that fight.
Staff has great defensive abilities and wand is great for making sure that your attack hits the enemy it needs to be. The only odd one out seems to be wand simply because it only effects a single attack roll(I.E. one enemy rather than one power), and because orb and staff both boost a stat that increases another defensive ability.
If you are pumping dex as a secondary to get that wand bonus up you are losing +1 defense every 4 levels and +2-3 at the start depending(as well as healing surges when you go Con, and lots of extra hit points on a surge if you are a dwarf).
But in the end, wand is more likely to get used each encounter and is likely to have the greatest effect (turning a miss into a hit on an important power), Orb is the strongest that will get used the least, and Staff is the one that keeps you alive the longest.
If you have balanced stats as a wizard you should probably pick up a second as you hit paragon tier. Orb/Wand is a nice combo since you can dual wield them easily, make all attacks through the wand(which can be a cheap-o wand while the Orb has some nice ability). And get a decent bonus to attack with those important powers that need to land(so that you can use your orb)
There is no odd man out for wizard specs. They are balanced pretty well.